1. Field of the Invention
The present invention relates to a development device, process unit, and image forming apparatus, and more particularly to a toner supply method for an image forming apparatus incorporating the development device and process unit.
2. Description of the Related Art
In electrophotographic image forming apparatuses, generally, a surface of an image bearer is charged uniformly, and then an exposure unit exposes the surface of the image bearer with, for example, a laser, according to image data, thus forming a electrostatic latent image thereon. Subsequently, a development device supplies developer (i.e., toner) to the surface of the image bearer with a development member such as a development roller, thereby developing the latent image into a toner image. The development device includes a toner containing compartment for containing a predetermined amount of toner for reliable image development, and an agitator that agitates the toner therein to give the toner a uniform electrostatic charge. The toner contained in the development device is consumed in image development, and the amount of toner remaining therein is detected by a toner detector. When the toner detector detects that the remaining toner amount is reduced to a predetermined amount, fresh toner is supplied to the development device from a replaceable toner container connected to the development device.
It is known that the charge properties of the toner inside the development device (hereinafter also “remaining toner”) are degraded gradually as the toner is agitated by the agitator. The deterioration in charge properties makes developing performance unstable. In this state, it is possible that image density fluctuates or toner scatters on backgrounds of images, which are ordinarily not exposed and should be kept free of toner. By contrast, charge properties of fresh toner (hereinafter also “supplied toner”) supplied from the toner container are not degraded. Consequently, when a large mount of fresh toner is supplied to the development device in which only a small amount of toner remains, the charge properties of the toner present in the development device change significantly after the supply of toner, affecting the developing performance. In view of the foregoing, several approaches, described below, have been tried to minimize fluctuations in charge properties of toner. For example, JP-2006-65079-A proposes controlling the amount of supplied toner to keep the amount of remaining toner substantially constant.
More specifically, in this approach, the amount of supplied toner is adjusted in response to the toner consumption as well as the amount of toner remaining in the development device to keep the amount of remaining toner substantially constant. Although this approach is effective as long as the toner is supplied constantly, if toner is not supplied to the development device timely and then a large amount of toner is supplied all at once, the charge properties of the toner therein can fluctuate dramatically.
More specifically still, although fresh toner is usually supplied from the toner container when the amount of toner remaining in the development device falls below the predetermined amount, fresh toner cannot be supplied when the toner container is empty. If development becomes unfeasible at the moment the toner container becomes empty, the printing job is suspended, which is inconvenient for users. Therefore, even when the toner container is empty, image development is generally continued using the toner remaining in the development device till a predetermined quantity. Additionally, when the toner container becomes empty, the apparatus generally indicates that the toner container should be replaced. However, when image development is continued with only the remaining toner after the toner container becomes empty, the remaining toner amount is smaller than usual. If the toner container is replaced in this state and fresh toner is supplied therefrom, the ratio of fresh toner to the remaining toner increases abruptly. As a result, developing performance becomes unstable, increasing toner scattering on the background and degrading the image quality.